Since recent developments in the fields of horizontal drilling and multistage fracturing many Exploration and Production (E&P) operators have experienced difficulties utilizing current technologies to mill or drill the seats out of a ball-type frac liner system. These ball frac sleeves prevent optimal productivity of the well and restrict the E&P company from entering the liner of the wellbore. Recent developments indicate that an intervention is required to remove the restrictions (balls and seats), to investigate inflow (production log or production evaluate), to restimulate the reservoir, and/or remove blockages such as sand or formation material.
Currently, the technology being used in these situations is typically conventional coiled tubing, water nitrogen mixtures, and mud motors with drill-bits or mills. These systems can increase the diameter of the liner by removing balls, seats, or other obstructions to achieve a maximum inner diameter of the liner. Current processes, however, create an over-balanced effect/position on the reservoir which in turn can lead to a loss of work-over fluids. A loss of work-over fluids will result in the undesired effect of frac proppant (sand) coming out of suspension and ‘sanding-in’ tools and tubing so that it cannot be removed. Sanding-in can result in the loss of tools, expensive fishing requirements, and potentially the loss of production from the well which can no longer be accessed. This over-balanced effect can also lead to formation damage resulting in reduced inflow from the formation or reservoir. The wellbore is often left with many of the solids from the seats, frac proppant (sand) and formation fines still present and not cleared from the liner. This limits the E&P company from getting the well to reach its maximum productivity and gather valuable data that would facilitate optimal development of a given field.
For E&P companies who are presently doing these operations, the cost and supply of nitrogen can seriously impact the economics and overall outcome. Safety is also major concern for E&P companies using current systems and the operations environment can be categorized as moderate to high risk. The reason for the safety concern is that the injection lines, coiled tubing, and return lines contain a highly compressible gas (typically nitrogen) and can be under extreme pressure. If a pressurized line or tubing is to part or break, the energy stored in the volume of the lined has to bleed off. This bleeding can cause the lines to whip uncontrollably until the energy has bled off. The uncontrolled lines can, in turn, contact and injure personnel and/or damage other equipment. The choice fluid/gas mixture typically used during current operations is low in density to maintain high velocity, this in turn is also known to wash out the surface iron (coiled tubing reel) and flow back vessel manifolds and connections.
Accordingly, there is a need to provide apparatuses and methods for clearing a wellbore that can overcome the short-comings of the prior art, such as unstable job costs, potential for formation damage, and unsafe work environments.